A German woman has died after being stung by a box jellyfish on a popular Thai resort island, the third reported fatality in Thailand in 14 months from one of the world's most toxic jellyfish.

The 20-year-old woman was taking an evening swim at a Koh Samui beach when she was stung by the jellyfish, whose sting can kill an adult within minutes, police Lt Col Thewet Pruemsut said.

A friend of the woman, also a German tourist, was stung on the hand by a jellyfish but survived.

The other two recent deaths from box jellyfish occurred on another popular island in the Gulf of Thailand, Koh Phangan, known for the notorious Full Moon Party. A Thai woman died there after being stung in August, while a five-year-old French boy died from a sting in August 2014.

Police on Samui met with business owners and municipal officials to discuss how to prevent future casualties from box jellyfish, which are not common to most parts of Thailand.

"Business owners are afraid that putting up warning signs would reduce the number of tourists, but we explained the necessity of this practice," Mr Thewet said.

He said that jellyfish are typically found in Thai waters during the rainy season, which runs roughly from June until October.

Thon Thamrongnawasawat, a marine biology expert and vice dean of the fisheries department at Bangkok's Kasetsart University, said that in the past seven years the numbers of box jellyfish in Thailand have increased due to several factors, including global warming, waste released from the shore that attracts jellyfish seeking food into shallow waters, and a decline in the numbers of sea turtles that eat jellyfish.

Unlike many other jellyfish, box jellyfish do not float on the sea surface but swim deeper and are harder to see, he said, making it advisable not to swim at night.

Box jellyfish have tentacles that can reach three metres (10ft) long. The sting is so excruciating that many victims go into shock and drown. Those who make it out of the water often die from the venom, which quickly attacks the heart and nervous system.